Tough maiden voyage - 1970 250 Automatic

[Number400]

CTH, thanks. I am sure the new plugs are sooty and I will change them again. The fuel flow has not been tested but when I pulled the line underneath the tank, it emptied the tank quickly and my new fuel filter primed in just a few times of cranking. I also just replaced the fuel line between the carbs as the old one sprung a leak last week. There was a decent amount coming out while just cranking the motor over. The bowls seem to have plenty fuel as every time I move the linkage, the accelerator pumps squirt fuel. There is no extra fuel in the barrels at idle.
Linkages are neutral and the free travel on the throttle linkage is moving first. I will check the fuel return valve more closely. It does push the pin up when the throttle is opened.

[BWhitmore]

The base setting for the air screws is to gently seat the screws and then turn out 1 1/2 turns. At one time there was a tool called Uni-Sync (?) that helped to synchronize the carburetors so that each were drawing the same about of air. This tool was commonly used to synch motorcycle carburetors.

[vwnate1]

Look on you tube for 'home made manometer', take the time to watch it and understand it, it's easy to make, you can use ATF or even water in the tubes....

[Number400]

Yup, I have a unisyn. To the left of the fuel/air screw is a small grub screw. What I cannot find in any manual is the function of that screw. I did find a post somewhere that it was the co2 adjustment.

[BWhitmore]

My guess is that the screw is for air/fuel mixture.

[Frank Reiner]

N4:

Many good thoughts here; each of the savants who are participating certainly has a picture in their mind of the carbs in question.
That said, perhaps you could bring the pictures into clearer focus; which carburetors are you working on - Solex or Zenith?

[vwnate1]

There should only be one mixture screw per carby, post a photo and someone will know what you're talking about .

I just looked back and didn't see where you mentioned what carbys and model number they are, Solex I assume ? .

The model # will be alpha - numeric and casted into the main body in raised text .

[Number400]

Originally Posted by Frank Reiner

N4:

Many good thoughts here; each of the savants who are participating certainly has a picture in their mind of the carbs in question.
That said, perhaps you could bring the pictures into clearer focus; which carburetors are you working on - Solex or Zenith?

The Zenith

[Number400]

I attached a parts list and circled the grub screw #3.

[vwnate1]

It looks like a blanking plug to me....

[Number400]

I had a chance to work on the car today. Front carb is now dribbling fuel down bore at idle and this makes the car stall after a running for a few minutes. I changed the float height washer in increments and there was no change with the dribbling. The 2mm is in there now and I will probably go back to the 1mm but I wanted to test every thickness.

The car starts easily and idles smoothly at ~900rpm.
The butterflies are seated and cracked just a touch via the short idle speed linkages.
The linkages between carbs and between the front carb are neutral.
Fuel are mixture screws are at a 1.5 turn out baseline.
The automatic chokes open evenly and fully as the car warms up.
The bowl vent pin on the front carb closes at 2k rpm.
The front carb has a 200-300 idle drop if I plug the small hole on the top of the carb.
Blocking the rear hole has only 100rpm drop and is slower to respond. I was working on cleaning out the horizontal passage today as that hole did appear to be blocked before.
QUESTION: One thing I noticed today was the secondary butterfly valves are opened a few millimeters via the small adjustment screws on the right side of the butterfly plate. I cannot find any info on if those should be opened slightly or not. Can anyone enlighten me on the settings for those?

Also, my rear accelerator pump is now stuck (again). I put in a new leather seal (replacing the blue neoprene one) and still cannot get it work reliably. I suspect that the springs are actually weakened after 49 years. I am going to see if I can find replacement pumps or even just springs.

Thanks much!

[Frank Reiner]

Originally Posted by Number400

I had a chance to work on the car today. Front carb is now dribbling fuel down bore at idle and this makes the car stall after a running for a few minutes. I changed the float height washer in increments and there was no change with the dribbling. The 2mm is in there now and I will probably go back to the 1mm but I wanted to test every thickness.


QUESTION: One thing I noticed today was the secondary butterfly valves are opened a few millimeters via the small adjustment screws on the right side of the butterfly plate. I cannot find any info on if those should be opened slightly or not. Can anyone enlighten me on the settings for those?

Given that changing the height of the float valve does not change the overflow, consider installing a fuel pressure regulator. Although Mother Benz intended the return valve to reduce fuel pressure at small throttle openings, it seem that the addition of a regulator improves that function. Two to three psi is sufficient.

The secondary throttle stop screws should be set to just keep the plates from sticking in the bores when closed.

[Number400]

Thanks Frank. I did install a Mr. Gasket regulator this morning but no luck, still dribbling at 3, 2.5, 2 and then the car does not run on 1psi. Oh well, it's nice to know that I can dial it in. Fuel was just pouring out of the main atomizer today, even at a high rpm, it was still peeing. I thought it would be more of a mist. I am going to pull the carbs off again and go through them one more time and start over with a slightly better understanding. I am sure I can get the rear idle circuit cleared to behave better and get that accel pump working better. The front carb, we will see...

Beaten down for today, going to go ride my bike.

[Benz Dr.]

I had the same problem on a 250S a few years ago and found that I had the wrong top cover gasket on one carb. There's a small hole drilled into the top cover which is supposed to regulate how much air is allowed to enter. Anyway, once I had the right gasket in place it ran like jack the bear.

I'll tell ya, I pissed around with those carbs for a week and could find nothing wrong, which was very discouraging. One of my guys read through the 280S work shop manual, figured out how everything was supposed to work and found the problem. Everything is regulated in some way or another on those carbs but I never suspected air correction problems.

[cth350]

if the drop in RPM happens for plugging either hole AND it's not the same, there's a mis-adjustment in the length of the linkages presumably. Make changes in tiny increments, like a quarter turn here and there and be prepared to undo any adjustment.

Also your idle should be more like 800rpm.

-CTH